1. Field of the Invention
The present invention relates to a laser oscillator that aims to stabilize pointing, namely a radiating position and a radiating direction of a laser beam.
2. Description of Related Art
Japanese Patent Publication (Kokoku) Nos. 63-64073 and 634-832 respectively disclose laser oscillators.
FIG. 20 is a perspective view diagramatically showing a laser oscillator disclosed in the Japanese Patent Publication (Kokoku) No. 63-64073.
Referring to FIG. 20, the oscillator radiates a laser beam 2. It has a duct 3 functioning as a laser medium gas passage, a pair of discharging electrodes 4a and 4b, a first laser beam reflecting unit 5a, a second laser beam reflecting unit 5b, a blower 6, a heat exchanger 8, and a casing 10 sealingly containing a laser medium gas under a low vacuum of about 1/10 normal atmosphere. A total reflector 26 is accommodated in the 2nd reflecting unit 5b. A partial reflector 32 is accommodated in the 1st reflecting unit 5a.
FIG. 21 is a front view showing a laser oscillator disclosed in the Japanese Publication (Kokoku) No. 64-832. FIG. 22 is a plan view of the oscillator of FIG. 21. FIG. 23 is a right side view of the oscillator of FIG. 21. FIG. 24 is an enlarged section taken along the line 24--24 and showing details of the oscillator of FIG. 23. FIG. 25 is an enlarged section taken along the line 25--25 and showing details of the oscillator of FIG. 23. FIG. 26 is an enlarged section taken along the line 26--26 and showing details of the oscillator of FIG. 23.
Referring to FIGS. 21 to 26, the oscillator has metal bellows 7, nuts 9, roller bearings 11, brackets 12, housings 13, spherical bearings 14, flanges 15, bolts 16 and 18, collars 17, base plates 20a and 20b, fixing seats 22 disposed on the casing 10, and supporting rods 24a, 24b and 24c. An inlet port 34 and an outlet port 36 are provided on the casing 10 for passing a cooling medium for cooling the heat exchanger 8. Pipes 35 and 37 connect the inlet port 34 and the outlet port 36 to the heat exchanger 8. An inlet port 40 and an outlet port 42 are provided on the casing 10 for passing a cooling medium for cooling a pair of discharging electrodes 4a and 4b. Pipes 41 and 43 connect the inlet port 40 and the outlet port 42 to the discharging electrodes 4a and 4b. A pipe 45 connects the discharging electrodes 4a and 4b. Fixing seats 22 fix the casing 10 to another structure.
An operation of the oscillator is described as follows.
In the casing 10, a pair of electrodes 4a and 4b produce discharge and excite the laser medium gas, and the blower 6 circulates the laser medium gas. The laser medium gas is cooled by the heat exchanger 8. The laser medium gas passes between the discharging electrodes 4a and 4b and is excited into a laser oscillatable state. The laser medium gas, that is raised to a high temperature by the discharge, flows into the duct 3, is cooled in the heat exchanger 8, and is circulated in an arrow direction A through the blower 6. A resonator mirror is constituted by the partial reflector 32 contained in the laser beam reflecting unit 5a and the total reflector 26 contained in the laser beams reflecting unit 5b, which are arranged in the longitudinal direction of the casing 10. An optical path produced by the resonator mirror passes through an excited area in which the laser medium gas is excited by discharging.
The laser beam reflected by the total reflector 26 reaches the partial reflector 32. Part of the laser beam which has reached the partial reflector 32 is permitted to be outputted to an exterior, and the rest returns to the total reflector 26 through an inverted route, thereby repeating the same process. The laser beam is amplified as it repeatedly passes the excited area as mentioned above, and is outputted from the partial reflector 32 to the exterior. The laser beam reflecting units 5a and 5b are mounted on a pair of base plates 20a and 20b which are held by the three supporting rods 24a, 24b and 24c. The bellows 7 connect the casing 10 and the base plates 20a and 20b positioned at right and left sides thereof to each other so as not to transmit external force to them.
In operating the laser oscillator, a cooling medium such as water needs to be supplied from a cooling unit or the like (not shown). The cooling medium introduced in the casing 10 via the inlet port 34 is supplied to the heat exchanger 8 through the pipe 35, and discharged out of the casing 10 through the outlet port 36. The cooling medium introduced in the casing 10 via the inlet port 40 enters into the lower discharging electrode 4b through the pipe 41, enters the upper discharging electrode 4a through the pipe 45, and is discharged out of the casing 10 through the pipe 43 and the outlet port 42. The casing 10 is fixed to a comparatively rigid structure such as a floor with foundation work or a frame of a power supply panel for supplying power to the laser oscillator, by use of the four fixing seats 22 provided at the lower part thereof.
Next, a supporting structure of the casing 10 and the base plates 20a and 20b is described. The base plate 20a is supported by the three supporting rods 24a, 24b and 24c. The connecting structures of the supporting rods 24a, 24b and 24c to the casing 10 are different. Namely, as shown in FIG. 25, the supporting rod 24a is held by the spherical bearing 14a in the housing 13 which is mounted on the casing 10. The base plate 20a is secured to one axial end of the supporting rod 24a by the nut 9. As shown in FIG. 26, the roller bearing 11 is attached to the supporting rod 24b, and the bracket 12 is fixed to the casing 10 by the bolt 18 and contacts linearly with the lower surface and the plane surface of the roller bearing 11. The collar 17 serves to position the roller bearing 11 when fastening the supporting rod 24b to the base plate 20a by the nut 9. As shown in FIG. 24, the supporting rod 24c is fixed to the base plate 20a by the nut 9 so as to be isolated from the casing 10. The three supporting rods 24a, 24b and 24c are made of a material of smaller linear expansion coefficient, such as invar, so that the base plates 20a and 20b do not lose their parallelism, even if there is a temperature change.
In operating the laser oscillator, first, the blower 6 is started. At the same time, a cooling medium generally of a temperature lower than a room temperature (e.g. 10.degree. C.) is supplied to the heat exchanger 8 and the discharging electrodes 4a and 4b, thereby causing the temperature and flow velocity of the cooling medium to assume a predetermined state at an entrance portion between the discharging electrodes 4a and 4b, so that the laser medium gas is effectively excited by discharge. After the cooling medium is supplied, the laser medium gas circulates at high speed when the blower 6 reaches a predetermined rotating speed, so that the laser medium gas has a predetermined temperature and a predetermined flow velocity. The time necessary for the preparatory operation corresponds to a rise time during which the rotating speed of the blower 6 reaches predetermined value. When the laser oscillator is ready, the laser is in a state capable of being oscillated.
The casing 10 changes its temperature gradually from the room temperature to a temperature near the cooling medium temperature, since the cooling medium pass the inlet ports 34 and 40 and the outlet ports 36 and 42. A time constant of the temperature variation is determined by a heat capacity of the casing 10. The constant is longer than the rise time of the laser oscillator, and changes gradually even after the laser oscillator has been ready. In case the cooling medium temperature is lower than the room temperature, a portion of the casing 10 specified by narrow slanted broken lines in FIG. 27 is partially cooled due to heat conduction from the inlet port 34, 40 or the outlet port 36, 42. Then, as shown in FIG. 27, only the upper part (part at the duct 3 side) is contracted due to linear expansion, thereby causing strain in the casing 10. As a result, the positions of the base plates 20a and 20b supported by the casing 10 are changed, so that a pointing of the laser beam is varied when oscillating laser. This phenomenon takes place at the time of rising and falling of the laser oscillator and has a comparatively long time constant. The strain amount of the casing 10 is determined by the difference between the cooling medium temperature and the ambient temperature. This phenomenon is called a "first mode of casing strain".
The laser medium gas made into a high temperature after passing the discharging electrodes 4a and 4b flows in the duct 3 when operating the laser oscillator, so that the duct 3 has a high temperature. Thus, the casing 10 adjacent to the duct 3 receives an entering heat due to heat radiation from the hot duct 3, so that part of the casing 10 facing the duct 3 experiences a gradual rise in its temperature and expands by heat expansion. Therefore, as shown in FIG. 28, there arises strain on the casing 10. As a result, the pointing of the laser beam is varied when oscillating the laser. This phenomenon is produced by ON and OFF operations of the laser oscillator and has a comparatively long time constant. The strain amount of the casing 10 is determined by a magnitude of a discharging input. This phenomenon is called a "second mode of casing strain".
In the oscillator of the background art, the three supporting rods 24a, 24b and 24c are supported on the base plates 20a and 20b in the same way, and there are no elements to mechanically determine the relative positional relationship between the casing 10 and the two base plates 20a and 20b secured to the three supporting rods 24a, 24b and 24c. This is because the supporting rod 24a is slidable on the inner surface of the spherical bearing 14a, as shown in FIG. 25, and the supporting rod 24b is slidable between the roller bearing 11 mechanically positioned by the collar 17 and the bracket 12. Accordingly, if a large acceleration is applied to such a sliding portion at the time of transportation or the like, the sliding portion possibly slides, and a stepped portion of the supporting rod 24a and the housing 13, or the stepped portion of the supporting rod 24b and the bracket 12, come into contact with each other. Thereby, though the casing 10 and the base plate 20a are isolated from each other before the above members are contacted, they will directly receive the strain force from the casing due to the above contact. Therefore, the base plates 20a and 20b will be directly affected by a large change of the casing 10, thereby preventing repeatability of the laser beam pointing.
Moreover, if the first mode of casing strain or the second mode of casing strain takes place, the distance between the two spherical bearings 14, which are fixed on the casing 10 that holds the supporting rod 24a, is relatively changed due to linear expansion (or contraction) by the temperature change of the casing 10. Therefore, the supporting rod 24a needs to slide on any one of the contact surfaces of the two spherical bearings 14a and 14b. Here, it is not certain whether the base plate 20a slides or the base plate 20b slides, since the plate having the least friction coefficient at that time will slide. Moreover, it is not assured that the slided side of the casing 10 due to the casing strain slides in the same amount when the casing 10 has gone back to a state before the strain. Thus, there is little repeatability of the relative position between the casing 10 and the base plates 20a and 20b, when repeating the rising and falling operations of the laser oscillator and ON and OFF operations of the laser. Accordingly, it is possible that the stepped portion of the supporting rod 24a and the housing 13 or the stepped portion of the supporting rod 24b and the bracket 12 sometime contact with each other, thereby causing factor of lack of repeatability of the laser beam pointing due to the contact.
It has been assumed that, if the parallelism of the resonant mirrors is maintained, the characteristics of the laser beam in laser operation does not change. However, it has been known that, if the relative positional relationship of the facing resonant mirrors, namely the relative positional relationship of the two base plates 20a and 20b and the casing 10 becomes offset, such characteristics as a beam mode and a converging performance of the laser beam, slightly change. Usually, there is brought forth a change in the relative lengths of the casing 10, which is made of a metal like iron, and the supporting rods 24a, 24b and 24c, which are made of a material of low linear expansion, each time that they are vibrated in transportation or the like or when the first or second mode of casing strain is generated and repeated. Moreover, the relative position between the base plates 20a and 20b and the casing 10 does not always go back to an original position when they return to their original lengths, respectively. It is possible that an offset of the relative position becomes larger gradually with time. Accordingly, the characteristics of the laser beam sometimes change with time. Thus, in case the laser oscillator is used for a laser work of high accuracy or the like, there arises a problem that the working effects will change in time.
The laser oscillator of background art is constructed as above, and requires improvement because the pointing stability of the laser beam will be lowered due to heat strain of the casing 10.